The presence of significant quantities of H.sub.2 S and CO.sub.2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons.
In one cyclic method currently attracting attention, the sour gas is contacted, preferably with a solvent-reactant system which comprises a regenerable reactant, to produce solid free sulfur which is recovered either prior or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which iron forms chelates with specified organic ligands.
In yet another process, e.g., that disclosed in U.S. Pat. No. 4,231,226 issued May 23, 1978, to Winkler, CO.sub.2 present in the gaseous stream is also removed by the use of a suitable selective absorbent.
Because these "cleanup" processes generally represent significant costs to manufacturing operations, any improvements in such processes which increase their efficiency may have great economic importance. For example, where ligands or chelates of polyvalent metals are employed, chemical degradation or decomposition of the ligand represents an important cost in the process, as well as requiring measures for decomposition bleed or removal and addition of fresh solution. Even in the case of a chelate such as that of nitrilotriacetic acid, ligand decomposition, over a period of time, requires attention to prevent build-up of decomposition products and consequent loss of efficiency.
Evidence exists that degradation of the ligand in such processes may be caused by or enhanced by regeneration of the reduced form of the chelated metal with oxygen or an oxygen-containing gas. The invention avoids this problem, and regenerates the ligand in a novel and efficient manner.